Thus, industrially, chlorine and sodium hydroxide, and sodium chlorate, are manufactured in electrolytic cells, each of them comprising several steel cathodes and several titanium anodes coated with a mixture of titanium and ruthenium oxides. As regards the preparation of chlorine and sodium hydroxide, the cells are generally fed with an electrolytic solution comprising approximately 200 to 300 g/l of sodium they generally comprise 50 to 250 g/l of sodium chloride.
However, these steel cathodes exhibit a relatively high overpotential in absolute value as cathodes for the reduction of water and also have an inadequate resistance to corrosion by dissolved chlorine.
The term “overpotential” is understood to mean the difference between the thermodynamic potential of the redox couple concerned (H2O/H2) with respect to a reference cathode and the potential actually measured in the medium concerned, with respect to the same reference electrode. The term “overpotential” will conventionally be used to denote the absolute value of the cathode overpotential.
Numerous cathodes have been proposed in order to overcome these disadvantages.
Thus, French Patent Application FR 2 311 108 discloses a cathode for which the substrate is a plate made of titanium, of zirconium, of niobium or of an alloy essentially composed of a combination of these metals and to which substrate a layer of metal oxide, essentially composed of an oxide of one or more metals chosen from ruthenium, rhodium, palladium, osmium, iridium and platinum and optionally an oxide of one or more metals chosen from calcium, magnesium, strontium, barium, zinc, chromium, molybdenum, tungsten, selenium and tellurium, is applied.
U.S. Pat. No. 4,100,049 discloses a cathode comprising a substrate made of iron, nickel or cobalt or of an alloy of these metals and a coating of palladium oxide and zirconium oxide.
European Patent Application EP 209 427 provides a cathode composed of an electrically conductive substrate made of nickel, of stainless steel or of mild steel carrying a coating composed of a plurality of layers of metal oxides, the surface layer being composed of an oxide of a valve metal, that is to say a metal chosen from Groups IVb, Vb and VIb of the Periodic Table of the elements, and the intermediate layer being composed of an oxide of a precious metal from Group VIII, that is to say ruthenium, rhodium, palladium, osmium, iridium and platinum.
The intermediate and surface layers can be composed of the oxide of the only metal concerned or of a mixed oxide of the metal concerned and of the second metal in a low proportion.
In Patent Application FR 2 797 646, the Applicant Company has provided an activated cathode composed of an electrically conductive substrate, either made of titanium or of nickel, coated with an intermediate layer of oxides based on titanium and on a precious metal from Group VIII of the Periodic Table of the elements and with an external layer of metal oxides comprising titanium, zirconium and a precious metal from Group VIII of the Periodic Table of the elements; the said coating being obtained by thermal decomposition of a solution of chloride or of oxychloride of these metals in ethanol or isopropanol.
Out of a concern for economy, it is increasingly desired to use cheaper substrates, such as substrates made of steel or of iron.